fix(nfs): add migration 0006 to repair fsid=0 exports

Migration 0005 (MigrateNFSClients) only runs once when first applied,
so any fsid=0 rows present before 0005 was applied never get repaired.

Migration 0006 triggers FixZeroFSIDs() which:
- Queries all nfs_exports rows with fsid=0
- Generates a new random fsid per row
- Updates the row

Version: 0.5.2

internal/db/migrations/0006_nfs_fix_zero_fsid.sql

@@ -0,0 +1,3 @@
+-- Repair NFS exports with fsid=0 (exportfs requires a non-zero fsid).
+-- Schema change: none.
+-- Data transformation is done by the Go hook FixZeroFSIDs in db.go.